Multiple reflector antenna



7 Sheets-Sheet 1 Fil ed May 16. 1966 n k l M S a m a J INVENTOR.

ATTORNEY.

Dec. 3,:1968 J. s. AJIOKA 3,414,904

MULTIPLE REFLECTOR ANTENNA Filed lay 16, 1966 7 Sheets-Sheet 2 J0me S.A" k F|g 3 INVENTORHO 0 BY. ax Wm ATTORNEY.

J. S. AJIOKA MULT IPLE REFLECTOR ANTENNA 7 Sheets-Shae :5

Filed llay 16. 1966 James S. Ajioka,

INVENTOR.

BY. 4 %m .ATTORNEY Dec. 3, 1968 .1. s. A JIOKA 3,414,904

MULTIPLE REFLECTOR ANTENNA Filed Bay 16. 1966 N 7 Sheets-Sheet 4 JamesS. A} ioku INVENTOR' ATTORNEY.

Dec. 3, 1968 J. 5. AJIOKA MULTIPLE REFLECTOR ANTENNA Filed may 16, 19667 Sheets-Sheet a James S. Ajioko,

INVENTOR.

' ATTORNEY,v

Dec. 3, 1968 J. S. AJIOKA MULTIPLE REFLECTOR ANTENNA 7 Sheets-Sheet 6Filed May 16, 1966 o k .m

BY. WM,

ATTORNEY 1386- 1963 J. s. AJIOKA MULTIPLE REFLECTOR ANTENNA 7Sheets-Sheet 7 Filed May 16. 1966 James S. Ajioku,

lNVENTOR.

ATTORNEY.

United States Patent 3,414,904 MULTIPLE REFLECTOR ANTENNA James S.Ajioka, Fullerton, Calif., assignor to Hughes Aircraft Company, CulverCity, Calif., a corporation of Delaware Filed May 16, 1966, Ser. No.550,483 18 Claims. (Cl. 343781) This invention relates to antennas, andmore particularly relates to a multiple reflector antenna of theCassegrain type which achieves highly eflicient, low noise operationover a wide frequency range.

In order to radiate highly directional beams of electromagnetic waveenergy into space, as well as to receive such beams, directionalantennas have been developed in which a feed element is located at thefocal point of a paraboloidal reflector. A problem encountered with suchantennas is that an appreciable amount of energy, termed spillover, isradiated or intercepted by the feed element directly withoutinterception by the paraboloidal reflector. This spillover energy notonly results in power loss and ineflicient operation, but interceptionof a portion of the spillover energy by the earth greatly increasesantenna noise.

An attempt to reduce the spillover energy by illuminating theparaboloidal reflective surface more efliciently has resulted in theapplication of the Cassegrain telescope principle to paraboloidalreflector antennas. In the Cassegrain antenna, a feed element located inthe general region of the vertex of a paraboloidal primary reflectorradiates signals to or intercepts signals from an intermediate secondaryreflector smaller than the primary reflector and which is locatedbetween the vertex and the focal point of the primary reflector. Thesecondary reflector, which usually assumes a hyperboloidalconfiguration, has a focal point essentially coincident with the focalpoint of the primary reflector.

Although the Cassegrain design is able to substantially reduce theamount of spillover energy past the primary reflector, a problem ofspillover past the secondary reflector exists. This secondary reflectorspillover also results in lossof power, and for antenna beams at lowelevation angles an appreciable portion of this spillover energy isintercepted by the earth, resulting in an increased noise temperature.

In addition, since the angular extent, termed feed angle, of the mainbeam portion of the radiation pattern from the feed element employed ina conventional Cassegrain antenna varies inversely with the frequency ofthe energy being radiated or received, such an antenna can be operatedwith optimum efliciency over only a very narrow frequency range. Forexample, if the frequency of operation is reduced below the optimumfrequency, the feed angle will be increased, resulting in excessivespillover. On the other hand, if the frequency of operation is increasedto above the optimum value, the feed angle will be decreased, whichresults in inefficient illumination of the reflector surfaces. Thus, thebandwidth over which conventional Cassegrain antennas are operable isseverely limited.

Accordingly, it is an object of the present invention to provide amultiple reflector antenna of the Cassegrain type which achieves highefliciency, low noise operation over a wider range of frequencies thanhas heretofore been afforded.

It is a further object of the present invention to provide a multiplereflector antenna of the Cassegrain type in which the amount ofspillover energy is substantially less than with prior art antennas ofthis type.

It is a still further object of the present invention to provide anantenna the noise temperature of which is sub- 3,414,904 Patented Dec.3, 1968 stantially less than that of prior art antennas when the antennais aimed at low elevation angles.

It is still another object of the present invention to provide anantenna feed arrangement for illuminating the secondary reflector of aCassegrain antenna with a feed pattern having a beamwidth which isessentially independent of frequency over at least an octave bandwidth.

In accordance with the objects set forth above, an antenna in accordancewith the present invention includes a primary reflector the intersectionof which with a given plane defines essentially a first parabola havinga focal point in the given plane, a secondary reflector the intersectionof which with the given plane defines essentially a second parabolahaving a focal point essentially coincident with the focal point of thefirst parabola, and a feed reflector the intersection of which with thegiven plane defines essentially a segment of an ellipse having first andsecond focal points. The first focal point of the ellipse is essentiallycoincident with the common focal points of the first and secondparabolas, and a feed element oriented so as to face the feed reflectoris disposed. essentially at the second focal point of the ellipse.

Additional objects, advantages and characteristic features of thepresent invention will become readily apparent from the followingdetailed description of preferred embodiments of the invention whenconsidered in conjunction with the accompanying drawings in which:

FIG. 1 is a sectional view illustrating an antenna in accordance withone embodiment of the present invention;

FIG. 2 is a sectional view illustrating the feed arrangement (feedelement and feed reflector) of the antenna of FIG. 1, includingradiation pattern diagrams and a feed reflector aperture distributiondiagram;

FIGS. 3(a), (b) and (c) are diagrams illustrating respective radiationpatterns from an ideal feed element, a conventional feed element, and afeed arrangement in accordance with the present invention;

FIGS. 4-8 are sectional views, similar to FIG. 1, illustratingrespective antennas in accordance with further embodiments of theinvention; and

FIG. 9 is a sectional view, similar to FIG. 2, showing an antenna feedarrangement in accordance with a still further embodiment of theinvention.

Referring with greater particularity to FIG. 1, a multiple reflectorantenna according to one embodiment of the invention may be seen toinclude a paraboloidal primary, or main, reflector 10 the surface ofwhich may be generated by rotating a parabola about an axis 12. Theparaboloidal reflector 10 has a vertex at 14 and a focal point at 16. Aparaboloidal secondary, or sub, reflector 18 substantially smaller thanthe primary reflector 10 is spaced from the primary reflector 10 alongthe axis 12 and has its convex surface facing the concave surface of thereflector 10. The secondary reflector 18, the surface of which may begenerated by rotating a parabola about the axis 12, has a focal pointessentially coincident with the focal point 16 of the primary reflector10 and has a vertex 19 which intersects the axis 12.

A tertiary, or feed reflector 20 is disposed between the primaryreflector 10 and the secondary reflector 18 to reflect energy emittedfrom a feed element 22 toward the secondary reflector 18, as well as toreflect energy from the reflector 18 toward the feed element 22. The

- feed reflector 20 has the configuration of a segment of axis of theellipsoid 24 at its end adjacent the primary reflector 10. The feedreflector 20 may be mounted on the primary reflector adjacent its vertex14 by means of a supporting element 28.

The feed element 22 may be a conventional source feed device, such as apyramidal or conical born for example, having its radiating aperturedisposed in a plane through the focal point 26 and perpendicular to theaxis 12. It is pointed out, however, that various types of feed elementsother than a horn feed, for example a multiaperture or multimodemonopulse tracking type feed, may be employed. Nevertheless, the feedelement should be selected so as to provide a radiation pattern havingwell defined sidelobes of desired amplitudes and widths, for example afirst sidelobe having a power level around 13 db below the maximum powerlevel of the main beam portion of the radiation pattern and a width halfthat of the main beam portion.

Electromagnetic wave energy, including the sidelobes, radiated by thefeed element 22 is reflected first by the feed reflector 20, then by thesecondary reflector 18, and finally by the primary reflector 10 fromwhich it emanates in a form of a highly directional beam traveling inthe direction of the axis 12. Conversely, electromagnetic beams receivedfrom space are reflected first by the primary reflector 10, then by thesecondary reflector 18, and finally by the feed reflector to the feedelement 22.

The manner in which the feed element 22 and the feed reflector 20function in accordance with the principles of the present invention toconsiderably reduce the spillover energy from that of conventionalCassegrain type antennas, as well as to provide an electromagnetic feedpattern having a beamwidth which is essentially independent offrequency, will now be discussed with reference to FIG. 2. As is shownin FIG. 2 the feed element 5 22 provides a radiation pattern (radiationintensity as a function of angular direction from the point of emission)designated generally by the numeral 30. The radiation pattern 30 has amain beam portion 32 and sidelobes 34 which are larger in relation tothe main beam portion than would normally be provided by a feed elementfor a Cassegrain antenna arrangement. The electromagnetic radiationemitted by the element 22 travels in a manner indicated by lines 35 tothe surface of the ellipsoidal feed reflector 20, from which it isreflected in a manner indicated by lines 36 to provide a desiredreflector aperture distribution (the electric field intensity in a planethrough the perimeter of the feed reflector 20 perpendicular to the axis12). This aperture distribution may be approximated mathematically bywhere J (ur) is a Bessel function, u is a factor proportional tofrequency, and 1" represents radial distance from the axis 12 in theaperture distribution plane. The projection of such an aperturedistribution in a plane through the axis 12 is indicated by curve 37 ofFIG. 2, the aperture distribution having a width W which is defined asthe cross-sectional dimension of that portion of the radi ationemanating from the reflector 20 due to the main beam portion 32 of theradiation pattern 30. It is pointed out that for a feed reflector havinga rectangular aperture instead of a circular aperture the resultantaperture distribution would be approximated mathematically by sin uxfrom the element 22, and thus a conventional Cassegrain antenna whichuses such a feed element to directly illuminate the secondary reflectorcan be operated with optimum efliciency over only a very narrowfrequency range. With the arrangement of the present invention, the feedangle 0 of the main beam portion 32 of the radiation pattern from thefeed element 22 will likewise vary inversely with frequency, causing theaperture distribution width W to vary inversely with frequency in termsof absolute dimension. However, since the wavelength of the emittedradiation is also a inverse function of frequency, the width W of theaperture distribution 37 is constant in terms of wavelength. Theresultant radiation pattern 40 from the feed reflector 20 (radiationintensity as a function of angular direction from the vertex of the feedreflector 20) is thus independent of frequency, enabling the antenna ofthe present invention to be operated with optimum efliciency over aconsiderable frequency range. The radiation pattern 40 may be seen topossess an essentially sector shaped main beam portion 42 which iscapable of more uniformly illuminating the secondary reflector 18 thanwould a radiation pattern main beam portion such as 32. The radiationpattern 40 may also be seen to possess sidelobe energy 44 which issubstantially smaller in relation to the main beam energy 42 than thesidelobe energy 34 is relative to the main beam portion 32 of theradiation pattern 30.

The improvement which the present invention achieves in providing a moreideal feed radiation pattern may be better appreciated by makingreference to FIG. 3 wherein the radiation intensity I is shown as afunction of angular direction 0 for an ideal feed element, aconventional feed element, and a feed element and reflector arrangementin accordance with the present invention. An ideal feed radiationpattern, illustrated by the curve 50 of FIG. 3(a), may be seen to havean essentially constant radiation intensity within a desired feed angle6 and no radiation outside of the angle 0 A feed radiation pattern usedto illuminate the secondary reflector of a conventional Cassegrainantenna is illustrated by the curve 52 of FIG. 3(b) wherein the energyradiated outside of the feed angle 0 i.e., the spillover energy, isdesignated by cross-hatching. The radiation pattern provided by feedelement 22 and feed reflector 20 of the antenna of the present inventionis shown by the curve 54 of FIG. 3(0), the spillover energy again beingdesignated by cross-hatching. It may be seen that the feed radiationpattern 54 produced in accordance with the present invention moreclosely resembles the ideal feed pattern 50 than does the conventionalfeed pattern 52, not only in providing a more constant radiationintensity within the feed angle 0 but also in considerably reducing thespillover energy outside of the feed angle 9 In accordance with afurther embodiment of the present invention, illustrated in FIG. 4, anoffset Cassegrain antenna arrangement may be provided. The embodimentshown in FIG. 4 is similar to that illustrated in FIG. 1, and elementsin the embodiment of FIG. 4 which correspond with elements in theembodiment of FIG. 1 are designated by the same reference numerals astheir counterpart elements except for the addition of the prefixnumeral 1. In the embodiment of FIG. 4, however, the major axis 113 ofellipsoid 124 is not coincident with axis 112, but rather is disposed ata predetermined angle a with respect to the axis 112 so that the axis113 intersects the primary reflector at a point 115 below the primaryreflector vertex 114. Feed reflector is mounted on the primary reflector110 adjacent the intersection point 115. Also, vertex 119 of thesecondary reflector 118 is located along the ellipsoid axis 113 ratherthan along the axis 112 which intersects the primary reflector vertex114. The embodiment of FIG. 4 is particularly useful when it is desiredto provide beams at low elevation angles because its design minimizesthe amount of secondary reflector spillover energy which intercepts theground, thereby reducing the antenna noise temperature.

In accordance with a still further embodiment of the present invention,illustrated in FIG. 5, an antenna arrangement having an offset feedreflector may be provided. The embodiment of FIG. 5 is also similar tothat of FIG. 1 and includes elements which, on account of theirsimilarity with elements in the embodiment of FIG. 1, are designated bythe same reference numerals as their counterpart elements except for theaddition of the prefix numeral 2. In the embodiment of FIG. 5 ellipsoid224 assumes the same location as the ellipsoid 24 of FIG. 1. However,the portion of the ellipsoid 224 defining the location of feed reflector220 is offset, i.e. is unsymmetrically disposed relative to the axis212. Also, the feed element 222 is angularly disposed so that a line 223perpendicular to the radiating aperture of the element 222 whichintersects the center of the feed reflector 220 is disposed at an angleB with respect to the axis 212.

Features of the embodiments of FIGS. 4 and 5 may be combined to producea still further embodiment, illustrated in FIG. 6, which employs both anoffset Cassegrain arrangement and an offset feed reflector. Since theembodiment of FIG. 6 is similar to the embodiments of FIGS. 4 and 5,elements in the embodiment of FIG. 6 which correspond with elements inthe embodiments of FIGS. 4 and 5 are designated by the same second andthird reference numeral digits as their counterpart elements; however,in the embodiment of FIG. 6 the numeral 3 is used as the first referencenumeral digit rather than the numeral 1 or 2. It should be noted that inan offset feed reflector arrangement the feed reflector need notintersect the major axis of the ellipsoid, and thus in the embodiment ofFIG. 6 feed reflector 320 is located entirely on one side of theellipsoid axis 313. Moreover, in the embodiment of FIG. 6 the angle 8between the ellipsoid axis 313 and line 323 perpendicular to theradiating aperture of the feed element 322 which intersects the centerof the feed reflector 320 is made essentially equal to 90.

In each of the foregoing embodiments the feed reflector has been shownas located in front of the primary reflector, i.e. between the primaryreflector and the secondary reflector. However, the principles of thepresent invention are also applicable to antennas in which the feedreflector is located behind the primary reflector, for example bymounting the feed reflector in a recess in the surface of the primaryreflector. Such a recessed antenna arrangement is illustrated in FIG. 7.The embodiment shown in FIG. 7 is similar to that illustrated in FIG. 1,and hence elements in the embodiment of FIG. 7 which correspond withelements in the embodiment of FIG. 1 are designated by the samereference numerals as their counterpart elements in the embodiment ofFIG. 1 except for the addition of the prefix numeral 4. In theembodiment of FIG. 7 the feed reflector is mounted in a recessed portion429 of the surface of the primary reflector 410. It is pointed out,however, that the feed reflector may also be located so that its vertexis flush with the surface of the primary reflector.

The recessed feed reflector arrangement illustrated in FIG. 7 may beutilized in combination with any of the other variations discussedabove. An example of an antenna arrangement combining the recessed feedreflector feature of FIG. 7 with the offset Cassegrain feature of FIG. 4is illustrated in FIG. 8 wherein elements are designated by the samesecond and third reference numeral digits as their counterpart elementsin FIGS. 4 and 7, except that the numeral 5 is used as the firstreference numeral digit rather than the numerals 1 or 4.

In accordance with still another embodiment of the present invention,illustrated in FIG. 9, the feed arrangement may employ a rearwardlyoriented feed element rather than a forwardly oriented feed element withrespect to the feed reflector. Since features of the embodiment of FIG.9 are readily illustratable by a drawing similar to FIG. 2, componentsof the feed arrangement of FIG. 9

and portions of the radiation pattern from the feed element thereofwhich have counterparts in FIG. 2 are designated by the same referencenumerals as such counterparts except for the addition of a primedesignation. In the embodiment of FIG. 9 the feed arrangement includes awaveguide portion 60 extending along major axis 12' of the ellipsoiddefining the location of feed reflector 20 and a reflector portion 62facing the feed reflector 20. The reflector portion 62 also faces endaperture 64 of the waveguide portion 60 and is spaced therefrom alongthe axis 12'. The reflector portion 62 may be attached to the waveguideportion 60 by means of rods 66, for example.

It is pointed out that while the specific embodiments of the presentinvention heretofore described employ primary and secondary reflectorshaving circular symmetry in planes perpendicular to the axisintersecting the vertex and the focal point of the primary reflector,the invention encompasses certain other antenna geometries which do nothave such circular symmetry. For example, the principles of theinvention could be utilized in constructing an antenna in which theprimary and secondary reflectors have the configuration of paraboliccylindrical segments, i.e., segments the intersections of which with aplane (such as the plane of the paper) perpendicular to the segmentsurfaces define parabolas (such as indicated at 10 and 18 in FIG. 1),having a common focal line (such as a line perpendicular to the plane ofthe paper and intersecting it at 16). The feed reflector in such anantenna arrangement could have the configuration of a segment of acylindrical elliptical surface, Le. a surface the intersection of whichwith a plane (such as the plane of the paper) perpendicular to thesurface defines an ellipse (such as indicated at 24 in FIG. 1), having apair of focal lines perpendicular to the plane of the ellipse andintersecting it at the focal points of the ellipse (such as at 16 and26). The common focal line of the parabolic cylindrical segments wouldbe essentially coincident with one of the focal lines of the cylindricalelliptical segment, and the feed element would be located along theother focal line of the cylindrical elliptical segment. Such acylindrical segment reflector could, of course, assume a pillbox, orcheese, configuration by making the parabolic cylindrical segments of aheight much smaller than the minimum segment-to-focal line distance andby providing enclosing plates at the ends of the parabolic cylindricalsegments in planes perpendicular to the cylindrical segments.

Thus, although the invention has been shown and described with referenceto particular embodiments, nevertheless various changes andmodifications which are obvious to a person skilled in the art to whichthe invention pertains are deemed :to lie within the spirit and scope ofthe invention as set forth in the appended claims.

What is claimed is:

1. An antenna comprising: a primary reflector the intersection of whichwith a given plane defines essentially a first parabola having a focalpoint in said plane, a secondary reflector the intersection of whichwith said plane defines essentially a second parabola having a focalpoint essentially coincident with the focal point of said firstparabola, a feed reflector the intersection of which with said planedefines essentially a segment of an ellipse having first and secondfocal points, said first focal point being essentially coincident withthe common focal points of said first and said second parabolas, and a'feed element disposed essentially at said second focal point and facingsaid feed reflector.

2. An antenna comprising: a first reflector the intersection of whichwith a given plane defines essentially a first parabola having a focalpoint in said plane, a second reflector substantially smaller than. saidfirst reflector and having a convexly curved surface facing said firstreflector, the intersection of said second reflector with said planedefining essentially a second parabola having a focal point essentiallycoincident with the focal point of said first parabola, a thirdreflector substantially smaller than said first reflector and having aconcavely curved surface facing said second reflector, the intersectionof said third reflector with said plane defining essentially a segmentof an ellipse having first and second focal points, said segment beinglocated nearer to said second focal point than to said first focalpoint, said first focal point being essentially coincident with thecommon focal points of said first and said second parabolas, and a feedelement disposed essentially at said second focal point and facing saidthird reflector.

3. An antenna according to claim 2 wherein said third reflector isdisposed between said first and said second reflectors.

4. An antenna according to claim 2 wherein said third reflector ismounted in a recessed portion of the surface of said first reflector.

5. An antenna according to claim 2 wherein an axis of said ellipsethrough said first and second focal points intersects said firstparabola essentially at its vertex.

6. An antenna according to claim 2 wherein an axis of said ellipsethrough said first and second focal points intersects said firstparabola at a point other than at its vertex.

7. An antenna according to claim 2 wherein said segment is symmetricallydisposed about an axis of said ellipse through said first and secondfocal points.

8. An antenna according to claim 2 wherein said segment isunsymmetrically disposed relative to an axis of said ellipse throughsaid first and second focal points.

9. An antenna according to claim 5 wherein said segment is symmetricallydisposed about said axis.

10. An antenna according to claim 6 wherein said segment issymmetrically disposed about said axis.

11. An antenna according to claim 9 wherein said third reflector isdisposed between said first and said second reflectors.

12. An antenna according to claim 9 wherein said third reflector ismounted in a recessed portion of the surface of said first reflector.

13. An antenna according to claim 5 wherein said segment isunsymmetrically disposed relative to said axis.

14. An antenna according to claim 6 wherein said segment isunsymmetrically disposed relative to said axis.

15. An antenna according to claim 2 wherein said feed element is a hornfeed element having an aperture disposed in a plane through said secondfocal point perpendicular to said given plane, with a line in said givenplane perpendicular to the plane of said aperture intersecting saidsegment at a point along the central portion thereof.

16. An antenna according to claim 2 wherein said feed element includes areflector portion facing said third reflector and a waveguide portionhaving an aperture spaced from and facing said reflector portion.

17. An antenna comprising: an essentially paraboloidal primary reflectorhaving a focal point, an essentially paraboloidal secondary reflectorhaving a focal point essentially coincident with the focal point of saidprimary reflector, a feed reflector having essentially the configurationof a segment of an ellipsoid having first and second focal points, saidfirst focal point being essentially coincident with the common focalpoints of said primary and said secondary reflectors, and a feed elementdisposed essentially at said second focal point and facing said feedreflector.

18. An antenna comprising: a first reflector of an essentiallyparaboloidal configuration having a focal point, a second reflector ofan essentially paraboloidal configuration substantially smaller thansaid first reflector, said second reflector having a focal pointessentially coincident with the focal point of said first reflector andbeing disposed with its convex surface facing said first reflector, athird reflector substantially smaller than said first reflector andhaving a concavely curved surface facing said second reflector, saidthird reflector having essentially the configuration of a segment of anellipsoid having first and second focal points, said segment beinglocated nearer to said second focal point than to said first focalpoint, said first focal point being essentially coincident with thecommon focal points of said first and said second reflectors, and a feedelement disposed essentially at said second focal point and facing said.third reflector.

References Cited UNITED STATES PATENTS ELI LIEBERMAN, Primary Examiner.

1. AN ANTENNA COMPRISING: A PRIMARY REFLECTOR THE INTERSECTION OF WHICHWITH A GIVEN PLANE DEFINES ESSENTIALLY A FIRST PARABOLA HAVING A FOCALPOINT IN SAID PLANE, A SECONDARY REFLECTOR THE INTERSECTION OF WHICHWITH SAID PLANE DEFINES ESSENTIALLY S ECOND PARABOLAD HAVING A FOCALPOINT ESSENTIALLY COINCIDENT WITH THE FOCAL POINT OF SAID FIRSTPARABOLA, A FEED REFLECTOR THE INTERSECTION OF WHICH WITH SAID PLANEDEFINES ESSENTIALLY A SEGMENT OF AN ELLIPSE HAVING FIRST AND SECONDFOCAL POINTS, SAID FIRST FOCAL POINT BEING ESSENTIALLY COINCIDENT WITHTHE COMMON FOCAL POINTS